Pneumatic tire

ABSTRACT

Disclosed is a pneumatic tire effectively controlling uneven wear occurring in the vicinity of a main groove even if a groove width of the main groove is widened due to change in tread radius during inflation. In the pneumatic tire, with regard to a zigzag-shaped main groove having the groove width widened during inflation among a plurality of main grooves provided on a tread surface, an inclination angle of a groove wall near the center with respect to the tread surface is made relatively large in a bent portion near the shoulder, an inclination angle of a groove wall near the shoulder with respect to the tread surface is made relatively large in a bent portion near the center, and regions where the inclination angles of the opposed groove walls are different from each other are uncontinuously arranged in the tire circumferential direction.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a pneumatic tire provided with aplurality of main grooves extended in a tire circumferential directionon the tread surface. More particularly, the present invention relatesto a pneumatic tire capable of controlling uneven wear occurring in thevicinity of the main grooves.

[0002] Generally, on the tread surface of the pneumatic tire, a singleor plural curvature radius (tread radius), as a curvature, in a tiremeridian direction is imparted based on the inner surface shape of a diemold. Meanwhile, in the pneumatic tire, the curvature radius on thetread surface tends to be changed during pressurization due to its innerstructures such as a belt layer buried inside the tread portion, treadrubber and the like. When a change in curvature radius on the treadsurface occurs in the above manner due to inflation, this change isabsorbed by the groove portion, resulting in the occurrence of aphenomenon such as bending of the tread portion at the groove bottom asa boundary. Particularly, in a rib tire provided with a plurality ofmain grooves extended in the tire circumferential direction, the abovebending phenomenon is significant.

[0003] As described above, when the tread portion is bent at the groovebottom as a boundary, the edge portion of the rib adjacent to the groovedoes not match the specified curvature radius of the tread surface, andthe ground-contacting pressure on the rib edge portion is significantlychanged. As a result, uneven wear occurs with the rib edge portion as astarting point, resulting in a growth of railway wear.

SUMMARY OF THE INVENTION

[0004] The object of the present invention is to provide a pneumatictire capable of effectively controlling uneven wear occurring in thevicinity of a main groove even in the case where the groove width of themain groove is widened due to a change in the tread radius duringinflation.

[0005] A pneumatic tire of the present invention for achieving theforegoing object is a pneumatic tire provided with a plurality of maingrooves extended in a tire circumferential direction on a tread surface,characterized in that, with regard to some of the main grooves among theforegoing plurality of main grooves, which have zigzag shapes swingingin a tire width direction and also whose groove width is widened duringinflation, in a bent portion near the shoulder, an inclination angle ofa groove wall near the center with respect to the tread surface is madelarger than an inclination angle of a groove wall near the shoulder, ina bent portion near the center, the inclination angle of the groove wallnear the shoulder with respect to the tread surface is made larger thanthe inclination angle of the groove wall near the center, and regionswhere the inclination angles of such opposed groove walls are differentfrom each other are arranged uncontinuously in the tire circumferentialdirection.

[0006] As a result of an intensive research on an occurrence mechanismof uneven wear such as railway wear, the inventors of the presentapplication found out that, in the case where the zigzag-shaped maingroove having the groove width widened due to the change in tread radiusduring the inflation, a ground-contacting pressure on an edge portionhaving a convex shape in a bent portion of the main groove is locallyincreased, and that the uneven wear occurs with that portion as astarting point.

[0007] In this connection, as described above, with regard to thezigzag-shaped main groove having the groove width widened due to thechange in tread radius during inflation, in the bent portion near theshoulder, the inclination angle of the groove wall near the center ismade relatively large, and in the bent portion near the center, theinclination angle of the groove wall near the shoulder is maderelatively large. In doing so, the rigidity of these regions is reduced,and the increase of the ground-contacting pressure on a rib edge portionis controlled. Hence, the uneven wear such as railway wear occurring inthe vicinity of the main groove can be effectively controlled.

[0008] More specifically, with regard to the main groove having thegroove width widened during inflation, it is preferable that, in thebent portion near the shoulder, while the inclination angle of thegroove wall near the center with respect to the tread surface is madelarger than 90°, the inclination angle of the groove wall near theshoulder is made smaller than 90°, and that, in the bent portion nearthe center, while the inclination angle of the groove wall near theshoulder with respect to the tread surface is made larger than 90°, theinclination angle of the groove wall near the center is made smallerthan 90°. As described above, by opposing the groove wall having theinclination angle smaller than 90° and the overhang-shaped groove wallhaving the inclination angle larger than 90°, the rigidity of the bothsides of the main groove can be effectively adjusted.

[0009] Moreover, in the event of arranging uncontinuously the regionswhere the inclination angles of the opposed groove walls are differentfrom each other (rigidity adjustment portions), for a zigzag pitch Z inthe tire circumferential direction of the main groove having the groovewidth widened during the inflation, the foregoing rigidity adjustmentportion should be provided in an area within 0.15 Z on both sides of thegroove bent point.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] For a more complete understanding of the present invention andthe advantages thereof, reference is now made to the followingdescription taken in conjunction with the accompanying drawings.

[0011]FIG. 1 is a cross-sectional view taken along a meridian direction,showing a pneumatic tire according to an embodiment of the presentinvention.

[0012]FIG. 2 is a development view showing a tread pattern of thepneumatic tire according to the embodiment of the present invention.

[0013]FIG. 3 is an enlarged plan view showing principal portions of thepneumatic tire according to the embodiment of the present invention.

[0014]FIG. 4 is a cross-sectional view of FIG. 3, taken along a lineIV-IV.

[0015]FIG. 5 is a cross-sectional view of FIG. 3, taken along a lineV-V.

[0016]FIG. 6 is a cross-sectional view of FIG. 3, taken along a lineVI-VI.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinbelow, description will be made in detail for theconstitution of the present invention with reference to the accompanyingdrawings.

[0018]FIG. 1 shows a pneumatic tire according to an embodiment of thepresent invention. A reference numeral 1 denotes a tread portion; anumeral 2 denotes a side wall portion; and a numeral 3 denotes a beadportion. Between a right-and-left pair of bead portions 3 and 3, acarcass layer 4 is bridged, and both end portions thereof in a tirewidth direction are wound up around bead cores 5, respectively from thetire inner side to the tire outer side. At the outer circumference sideof the carcass layer 4 in the tread portion 1, a plurality of beltlayers 6 are buried.

[0019] As shown in FIG. 2, on a tread surface 1 a, a plurality ofstraight-shaped main grooves 7 a extended in the tire circumferentialdirection and a plurality of zigzag-shaped main grooves 7 b extended inthe tire circumferential direction while swinging in the tire widthdirection are formed. The main grooves 7 a are located at the treadcenter side, and the main grooves 7 b are located more to the shoulderside than the main grooves 7 a. These main grooves 7 a and 7 b definethe plural columns of ribs 8. It should be noted that side grooves,sipes and the like, which are extended in the tire width direction, maybe provided on the tread surface 1 a according to needs.

[0020] In the above pneumatic tire, due to the inner structure such asthe belt layer 6 buried inside the tread portion 1, tread rubber and thelike, the curvature radius of the tread surface 1 a is changed duringinflation, and the tread portion 1 is bent mainly at the groove bottomof the main groove 7 b on the shoulder side as a boundary. As a result,the groove width of the main groove 7 b is slightly widened compared tothat before the inflation. It is possible to intentionally design thetire inner structure causing such a bending phenomenon based on the beltlayer 6, the tread rubber and the like. For example, if thecircumferential rigidity in the vicinity of the tread shoulder isrelatively increased, growth of the outer circumference in the vicinityof the center is relatively increased during the inflation.

[0021] In the main groove 7 b having the groove width widened during theinflation, as shown in FIG. 3, the inclination angle of the groove wallW₁ near the center is made larger than the inclination angle of thegroove wall W₂ near the shoulder in the bent portion near the shoulder,and in the bent portion near the center, the inclination angle of thegroove wall W₂ near the shoulder is made larger than the groove wall W₁near the center. In FIG. 3, solid lines of the main groove 7 b indicateoutlines thereof on the tread surface, and dotted lines indicateoutlines thereof at the groove bottom.

[0022] FIGS. 4 to 6 are cross-sectional views of FIG. 3, taken alonglines IV-IV, V-V and VI-VI, respectively. As shown in FIG. 4, in thebent portion of the main groove 7 b near the shoulder, the inclinationangle α₁ of the groove wall W₁ near the center with respect to the treadsurface 1 a is made larger than the inclination angle β₁ of the groovewall W₂ near the shoulder with respect to the tread surface 1 a. Morespecifically, in the bent portion near the shoulder, the inclinationangle α₁ of the groove wall W₁ near the center is set larger than 90°,and the inclination angle β₁ of the groove wall W₂ near the shoulder isset smaller than 90°.

[0023] As shown in FIG. 5, at an arbitrary position between the bentportions of the main groove 7 b, the inclination angle α₂ of the groovewall W₁ near the center with respect to the tread surface 1 a and theinclination angle β₂ of the groove wall W₂ near the shoulder withrespect to the tread surface 1 a are equal to each other. At such thearbitrary position between such bent portions, both the inclinationangle α₂ of the groove wall W₁ near the center and the inclination angleβ₂ of the groove wall W₂ near the shoulder are set smaller than 90°. Itshould be noted that the inclination angles α₂ and β₂ may be somewhatdifferent from each other within a range smaller than 90°.

[0024] As shown in FIG. 6, in the bent portion of the main groove 7 bnear the center, the inclination angle β₃ of the groove wall W₂ near theshoulder with respect to the tread surface 1 a is made larger than theinclination angle α₃ of the groove wall W₁ near the center with respectto the tread surface 1 a. More specifically, in the bent portion nearthe center, the inclination angle β₃ of the groove wall W₂ near theshoulder is set larger than 90°, and the inclination angle α₃ of thegroove wall W₁ near the center is set smaller than 90°.

[0025] As shown by a chain and dotted line of FIG. 1, in the abovepneumatic tire, the tread portion 1 is bent mainly at the groove bottomof the main groove 7 b on the shoulder side as a boundary during theinflation, and the groove width of the main groove 7 b is slightlywidened compared to that before the inflation. Therefore, as shown inFIG. 2, the ground-contacting pressure on the edge portion E with aconvex shape in the main groove bent portion tends to be locallyincreased. However, as described above, the inclination angle α₁ of thegroove wall W₁ near the center is made relatively large in the bentportion of the main groove 7 b near the shoulder, and the inclinationangle β₃ of the groove wall W₂ near the shoulder is made relativelylarge in the bent portion near the center, whereby the rigidity in thevicinity of the edge portion E is reduced to control the increase of theground-contacting pressure on the rib edge portion. Hence, the unevenwear such as railway wear occurring in the vicinity of the main groovecan be effectively controlled.

[0026] In the above pneumatic tire, the rigidity adjustment portions 9,in which the inclination angles of the opposed groove walls W₁ and W₂ ofthe main groove 7 b having the groove width widened during the inflationare made to differ from each other, are uncontinuously arranged in thetire circumferential direction as shown in FIG. 3. More specifically,the portions whose rigidity is to be lowered are locally arranged, andthe initial rigidity of the portions other than the above is notchanged, thus enabling the uneven wear to be effectively controlled. Inthe event of uncontinuously arranging the rigidity adjustment portions 9in the tire circumferential direction, for the zigzag pitch Z in thetire circumferential direction of the main groove 7 b having the groovewidth widened during the inflation, the rigidity adjustment portions 9is provided in an area within 0.15 Z on both sides of the groove bentpoint. If the rigidity adjustment portion 9 is out of the area within0.15 Z on both sides of the groove bent point, evenness of theground-contacting pressure is lowered, thus facilitating the uneven wearto occur. It should be noted that, in the case where the zigzag-shapedmain groove 7 b is not provided with obvious bent points, it issatisfactory if outlines of the main groove are extended toward the bentportion, and an intersection point of these extended lines is assumed asa bent point.

[0027] In the present invention, the number of main grooves provided onthe tread surface is not particularly limited, and it is satisfactory ifthe groove width of at least a part of the zigzag-shaped main grooves iswidened during the inflation.

[0028] Example

[0029] Tires of the present invention and conventional tires, which havethe same tire size of 11.00R22.5 and different tread patterns, wereproduced.

[0030] Tire of the Present Invention

[0031] As shown in FIG. 1, in the pneumatic tire provided with aplurality of main grooves extended in the tire circumferential directionon the tread surface, with regard to the main grooves having zigzagshapes swinging in the tire width direction and the groove width widenedduring inflation, the inclination angle β₁ of the groove wall near theshoulder is set at 80° while the inclination angle α₁ of the groove wallnear the center is set at 100° in the bent portion near the shoulder,and in the bent portion near the center, the inclination angle α₃ of thegroove wall near the center is set at 80° while the inclination angle β₃of the groove wall near the shoulder is set at 100°. The regions wherethe inclination angles of the opposed groove walls are different fromeach other are uncontinuously arranged in the tire circumferentialdirection.

[0032] Conventional Tire

[0033] In FIG. 1, each inclination angle of the both groove walls of thezigzag-shaped main grooves having the groove width widened during theinflation are set at 80°.

[0034] Uneven wear property of these test tires was evaluated. As amethod for evaluating the uneven wear property, two pieces of either thetires of the present invention or the conventional tires were attachedas front tires to each of eight trucks, and were made to run mainly on ahighway for a sixty thousand kilometers. Thereafter, occurrence of theuneven wear on the tread surface of each tire was observed.

[0035] As a result, with regard to the conventional tires, the railwaywear occurred in six tires among sixteen tires. On the other hand, withregard to the tires of the present invention, the railway wear occurredonly in two tires among sixteen tires, including the one only with asign thereof.

[0036] As described above, according to the present invention, in thepneumatic tire provided with the plurality of main grooves extended inthe tire circumferential direction on the tread surface, even if thegroove width of the main grooves is widened due to the change in treadradius during inflation, the uneven wear occurring in the vicinity ofthe main grooves can be effectively controlled.

[0037] Although the preferred embodiment of the present invention hasbeen described in detail, it should be understood that various changes,substitutions and alternations can be made therein without departingfrom spirit and scope of the inventions as defined by the appendedclaims.

What is claimed is:
 1. A pneumatic tire provided with a plurality ofmain grooves extended in a tire circumferential direction on a treadsurface, wherein, with regard to a main groove having a zigzag shapeswinging in a tire width direction and also a groove width widenedduring inflation among said plurality of main grooves, an inclinationangle of a groove wall near the center with respect to the tread surfaceis made larger than an inclination angle of a groove wall near theshoulder in a bent portion near the shoulder, the inclination angle ofthe groove wall near the shoulder with respect to the tread surface ismade larger than the inclination angle of the groove wall near thecenter in a bent portion near the center, and regions where theinclination angles of the opposed groove walls are different from eachother are uncontinuously arranged in the tire circumferential direction.2. The pneumatic tire according to claim 1, wherein, with regard to saidmain groove having the groove width widened during inflation, theinclination angle of the groove wall near the shoulder with respect tothe tread surface is set smaller than 90° while the inclination angle ofthe groove wall near the center is set larger than 90° in the bentportion near the shoulder, and the inclination angle of the groove wallnear the center with respect to the tread surface is set smaller than90° while the inclination angle of the groove wall near the shoulder isset larger than 90° in the bent portion near the center.
 3. Thepneumatic tire according to claim 1, wherein, for a zigzag pitch Z inthe tire circumferential direction of said main groove having a groovewidth widened during inflation, said region where the inclination anglesof the opposed groove walls are different from each other are providedwithin an area of 0.15 Z on both sides of a groove bent point.
 4. Thepneumatic tire according to claim 2, wherein, for a zigzag pitch Z inthe tire circumferential direction of said main groove having a groovewidth widened during inflation, said region where the inclination anglesof the opposed groove walls are different from each other are providedwithin an area of 0.15 Z on both sides of the groove bent point.